SG_USA_July_2021

BEAT THE HEAT How Conduction, Convection, Evaporation and Radiation Can Help Cattle Avoid Heat Stress SANTA GERTRUDIS Product ion

By Randy L. Stanko, Ph.D., Texas A&M University-Kingsville E xactly how do beef cattle gain or lose body heat? If we raise cattle in the southern United States, we must understand that livestock are

Air movement can also help with the third principle of heat transfer – evaporation – a process by which a liquid is transformed into vapor. This will require energy (heat). If you spray perfume or cologne on your skin, it feels cool. This is because the alcohol in perfume (similar to ethanol) has an evaporation rate that is greater than water and is using energy to become a vapor. Sweating is very effective in cooling horses and humans and is a normal physiological response to an increase in body temperature. Unfortunately, sweating is not the best way to reduce body heat in cattle, although Brahman-influenced cattle do it best. Unlike dogs, panting is not a normal physiological response to elevated body temperature for cattle. If you see a cow with her head down, neck bowed and panting, she is in severe heat stress and needs to be cooled down quickly with cool, flowing water. Other ways cattle can increase heat loss is through reduced feed intake and reduced typical daily activi- ties, increased peripheral blood flow and hair shedding. Radiation is the process of heat transfer between two objects that are not touching. Heat will always move from a warmer object to a cooler object. Therefore, cattle can either gain or lose body heat through radiation. Without sunlight, summer nights are normally cooler in temperature than summer days. The most efficient and critical way for cattle to lose the body heat load built up during summer days is simply exposure to a summer night. Soon after sundown, all cattle will radiate body heat into the atmosphere until sunrise. In addition to conduction, convection, evaporation and radiation, providing cattle with plenty of cool drinking water is an extra tool that will help them avoid heat stress.

exposed to and expected to perform under less-than-ideal weather condi- tions. Hopefully, our cattle (American breeds) can perform better than other breeds in this environment because they are genetically adapted, which means they have undergone accli- matization. This genetic adaptation

has occurred over a long time as we have bred and selected cattle to have distinct characteristics that help them better cope with extended periods of elevated heat and humidity. Characteristics such as a slick hair coat, increased skinfolds to increase surface area, dark skin pigmentation and the abil- ity to sweat more than cold-weather-adapted counterparts. Nonetheless, it makes some sense to review the principles of how cattle can gain or lose body heat to their surround- ings. If well understood, then management practices can be adjusted to facilitate an animal’s ability to deal with common or exceptional weather extremes. Conduction occurs when heat flows from an animal to something else that the animal is touching. If a cow is lying on a concrete floor in the winter, she will lose body heat to the concrete. If a winter-born calf is lying on warm, dry hay, it can gain body heat from conduction. Thus, during summer heat, cattle will seek shaded soil and sometimes even water. Usually, standing in stock pond water in the southern United States is an activity often reserved for those cold-weather-adapted counterparts, unless the heat index is extreme. Convection is the process of heat transfer arising from air or water movement. In a severe case of heat stress (human, pet or cattle), cool water flowing across the body will reduce body temperature due to convec- tion. A light to moderate breeze is always a welcome event to both man and beast during summer heat. If cattle can experience air temperatures less than normal body temperature (101.5 degrees F) and if that air is also moving, they will reduce body heat through convection. The greater the air velocity, the greater the heat transfer from cattle to the surrounding air. Again, shade options plus air movement are important.

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